Polymer blends may be made by a variety of methods. A flexible but expensive off-line process of making polymer blends uses solid polymers as starting materials, typically outside the polymerization process that produced the polymer blend components. The polymer blend components are typically first melted or dissolved in a solvent and then blended. These processes are known as melt-blending and off-line solution blending, respectively. In melt blending, the solid, often pelletized or baled, polymer blend components are first melted and then blended together in their molten state. One of the difficulties presented by melt blending is the high viscosity of molten polymers, which makes blending of two or more polymers difficult and often imperfect on the molecular level. In solution off-line blending, the solid, often pelletized, polymer blend components are first dissolved in a suitable solvent to form a polymer solution, and then two or more polymer solutions are blended together. After blending, solution blending requires the extraction of solvent from the blend and drying of the blended polymer. Solution blending can overcome the viscosity issue associated with melt blending, but is expensive due to the need for redissolving the polymer blend components and due to the cost of solvent handling.
The common feature of both melt blending and off-line solution blending is that the polymer blending components are made in separate plants and the solid polymers then are reprocessed either in a molten or in a dissolved state to prepare the final polymer blend. In fact, these off-line blending processes are often operated by so-called compounders, generally independent of the manufacturers of the polymer blend components. These processes add considerable cost to the cost of the final polymer blend. The production and full polymer recovery in separate plants and subsequent reprocessing increases the costs of producing such blends because of the need for duplicate polymer recovery lines and because of the need for separate blending facilities and the energy associated with their operations. Off-line solution blending also requires extra solvent, and facilities for polymer dissolution and solvent recovery-recycle. Substantial reprocessing costs could be saved if the polymer blends could be made in one integrated polymerization plant in-line, i.e. before the recovery and pelletizing of the solid polymer blend components.
The disadvantage of a separate polyolefin blending plant associated with the melt blending and off-line solution blending processes is alleviated with the prior art method of in-line solution blending of polymers using a series reactor configuration. Utilizing the series reactor configuration, product blending may be accomplished in the solution polymerization reactor itself when the effluent of the first solution polymerization reactor is fed into the second reactor operating at different conditions with optionally different catalyst and monomer feed composition. Referring to the two-stage series reactor configuration of FIG. 1 (prior art), the two different polymers made in the first and second reactor stages are blended in the second stage yielding a blended polymer product leaving the second reactor. Such reactor series configuration may be further expanded into more than a two-stage series configuration (three or more reactors in series). Generally, a series of n reactors may produce a blend with as many as n components or even more present in the effluent of the last reactor. Note that in principle, more than n components may be produced and blended in n reactors by, for example, using more than one catalyst or by utilizing multiple zones operating at different conditions in one or more reactors of the series reactor cascade. While mixing in the downstream reactor(s) provides good product mixing, particularly when the reactors are equipped with mixing devices, e.g., mechanical stirrers, such series reactor configuration and operation presents a number of practical process and product quality control problems due to the close coupling of the reactors in the cascade. One of the most important difficulties in commercial practice is ensuring proper blend and monomer ratios to deliver consistent blend quality. Additional complications arise when the blend components have different monomer compositions, particularly when they have different monomer pools, such as in the case of blending different copolymers or in the case of blending homo- and copolymers. Since the monomer streams are blended, there is no option for their separate recovery and recycle mandating costly monomer separations in the monomer recycle lines.
The above-outlined issues with series reactor operations are apparent to those skilled in the art of chemical engineering. These difficulties are particularly significant in polymerization because unlike in small-molecule syntheses, reactor conditions determine not only reactor productivities related to product blend ratio, but also product properties related to controlling the quality of the polymer blend components. For example, FIGS. 2 and 3 show how reactor temperature and pressure affect polymer properties of fundamental importance, such as molecular weight (MW) and melting behavior. Surprisingly, we found that monomer conversion in the reactor also influences these critical product attributes (see FIG. 4). Since in a series reactor cascade the effluent of an upstream reactor flows into the next downstream member of the reactor cascade, the residence time, catalyst concentration, and monomer composition and thus monomer conversion in the downstream reactor cannot be adjusted independently of the operating conditions (particularly of the flow rate) of the upstream reactor. Because of this close and inherent coupling of operating regimes in the reactors of the series cascade, the correlations depicted in FIGS. 2, 3, and 4 further reduce the controllability, flexibility, and thus the usefulness of the in-line blending method in a series reactor configuration. Ultimately, this greatly reduces the number of blend products that can be made in such a series reactor cascade and makes the blend quality difficult to control.
Applying parallel reactors can overcome the disadvantages related to the direct coupling of the polymerization reactors in an in-line polymer blending applying series reactors. While production flexibility is increased, a parallel reactor arrangement necessitates the installation of blending vessels increasing the cost of the process. As disclosed in U.S. Patent Application No. 60/876,193 filed on Dec. 20, 2006, herein incorporated by reference in its entirety, an improved in-line process for blending polymers has been developed to improve blend quality and reduce the capital and operating costs associated with a combined polymerization and blend plant.
One problem associated with the fluid phase in-line blending process utilizing two or more parallel reactor trains and the two or more high-pressure separators fluidly connected to the two or more reactor trains configured in parallel is that the polymerization processes do not convert 100% of the feed monomers in a single pass through the parallel reactor trains. Because the monomers are of high value, the unconverted monomer streams are typically recovered by separating them from the polymeric products and recycled back to the polymerization reactor bank. However, when the monomer-rich phase from a high-pressure separator contains two or more different monomers, separation and recycle become more challenging and complex. In particular, when the monomer-rich phase from a high pressure separator contains a mixture of a monomer stream from a homopolymerization parallel reactor train and a comonomer stream from a copolymerization parallel reactor trains, recycle of the monomer and comonomer streams becomes particularly challenging due to the fact that the monomer composition of the recycle stream is often too far from the desired reactor feed composition. This imbalance can be best quantified by the excess monomer component flow present in the recycle monomer stream. In order to allow the rebalancing of the monomer feed composition, the monomers present in the combined recycle stream are typically split into individual pure monomer feeds. These individual monomer recycle streams then are combined with the make up monomer streams at a controlled rate to provide for the reactor feeds at the required monomer feed rate and composition. Such monomer separation trains, however, are capital intensive and expensive to operate. They are particularly expensive for light olefinic monomer recycle streams comprising aliphatic olefins of two to four carbon atoms due to their low boiling points and thus due to the need for cryogenic distillation. Hence, there is a need for new inventive processes that minimize or even eliminate such expensive monomer separations in the monomer recycle train of in-line blending processes.
There are a variety of possible options for recovering and recycling the monomers. One option, outside the scope of the present disclosure, recovers the monomers in separate trains that do not contact each other and combine the monomer and comonomer streams before, during, or after the monomer separation from the polymers. The monomer and comonomer streams are kept separate and may be recycled to their corresponding polymerization reactor trains without concerns about contaminating the feed of the homopolymerization train by the comonomer of the copolymerization train. While this option eliminates the need for and the cost associated with separation and recovery of the individual monomers, it requires parallel feed-product separation trains for each reactor train. It also handles and blends the undiluted, highly viscous polymer blend components in their molten state, as opposed to their diluted fluid state. Both of these requirements increase cost and decrease blend quality.
FIG. 5 depicts the process flow diagram of another prior art monomer recycle process used in conjunction with fluid phase in-line blending of polymers. Referring to FIG. 5, the reactor effluents (1) and (2) from the two parallel reactor trains (1) and (2) are blended in the single separator-blender that also separates a monomer-rich phase from a polymer-rich phase. The monomer-rich phase emerging from the top of the separator-blender comprises a mixture of the monomers of reactor train (1) and the monomers from reactor train (2). This combined monomer stream is then separated into its individual monomer components before those components are recycled to the proper reactor trains of the in-line polymer blending process. This prior art monomer separation and recycle method provides ultimate flexibility in the recycle train since each monomer component is recovered individually and can be directed at the desired rate to the appropriate reactor train(s). However, it also applies a complex and expensive monomer separation train. A recycle process that can operate without the full recovery of each individual monomer components would be cost advantaged, simpler to operate and maintain, and thus desirable.
Besides separating the monomer components present in the monomer recycle stream emerging from the separator-blender, heavier impurities, such as solvents used in the catalyst feed, oligomeric side products, excess catalyst killer(s), etc. need to be separated and purged from the monomer recycle stream in order to prevent their buildup in the reactor trains. Again, referring to FIG. 5, separation tower (1) separates the combined monomer-comonomer stream from solvents, oligomers and other heavies, which emerge from the bottom of tower (1). Separation tower (2) further separates the monomer-containing stream emerging from the top of tower (1). Monomer (1) emerges from the bottom and monomer (2) emerges from the top of tower (2). The monomer (1) bottom stream from separation tower (2) is recycled back to polymerization parallel reactor train (1) and the monomer (2) top stream from separation tower (2) is recycled back to polymerization parallel reactor train (2). It should be understood that in the case of monomer recycle streams comprising more than two monomers, more than two separation towers may be necessary to recover the individual monomer components. These chilled separation towers and associated hardware are expensive to install from a capital equipment standpoint and expensive to operate from utility standpoint. The separation towers also increase maintenance costs associated with the polymerization-blending plant. In addition, the separation process step also increases the overall in-line blending process complexity and thus the probability of process upsets. Therefore, there is a need for an improved method that can reduce the complexity and cost of monomer recycle related to the in-line polymer blending process.